The present invention relates, in general, to temperature control equipment techniques in chemical reactors and, in particular, to a new and useful temperature control system for an olefin oxidation reactor which regulates the rate of coolant flow to maintain the reactor temperature within a desired temperature range.
Various techniques and systems are known for controlling chemical reactors.
U.S. Pat. No. 3,080,219 to Harvey Jr. discloses a control system for a polymerization reactor. This system is applicable to stirred tank reactors where temperatures are uniform throughout the reactor due to mixing.
U.S. Pat. No. 4,132,530 to Schwimmer discloses a temperature control system for an exothermic or endothermic reaction. Schwimmer provides a plurality of temperature sensors distributed along the reactor axis for measuring maximum reactor temperatures used in a control scheme.
Other references which are relevant in understanding the present invention are: U.S. Pat. No. 3,373,218 to Schuman, U.S. Pat. No. 4,132,529 to Schwimmer, U.S. Pat. No. 4,187,542 to Ball et al. and U.S. Pat. No. 4,257,105 to Stewart et al. All these references disclose various control techniques for chemical reactors.
In an ethylene oxide manufacturing process, ethylene and oxygen or air is mixed and fed to an isothermal multitubular reactor. Ethylene is oxidized into ethylene oxide in the presence of a catalyst and carbon dioxide and water are produced as by-products. Reactor temperature control objectives are:
Operation at the most economical temperature; PA1 Operation within a safe zone; PA1 Maximum conversion to ethylene oxide while minimizing by-products; PA1 Reduced consumption of coolant; PA1 Avoidance or elimination of unsafe operation; and PA1 Reduced operator attention. PA1 a reactor runaway condition; PA1 catalyst poisoning; PA1 increased coolant demand; PA1 an unsafe operating situation; and/or PA1 increased operator attention.
Reactor temperature control is of key significance because of the following factors:
1. The most economical temperature for oxidation is one at which the highest conversion to ethylene oxide occurs rather than to by-products. PA0 2. Catalyst selectivity increases as the reaction temperature is lowered while ethylene conversion increases with increasing reactor temperature. Thus, temperature requirements for high selectivity and high conversion are opposed. This results in a narrow temperature range for reactor operation. PA0 3. Increase in reaction temperature produces two effects: (1) overall rate of ethylene oxidation increases, and (2) catalyst selectivity to ethylene oxide decreases such that relatively more ethylene is converted into carbon dioxide and water. Moreover, heat generation increases by the fact that more ethylene is oxidized and overall reaction becomes less selective. Consequently, increase in temperature may result in:
Hence, neither a temperature rise nor a temperature drop is desirable.
In the state of the art system, reactor temperature control system is based on manipulating coolant flow rate. Its set point is directly based upon average reactor temperature. These control schemes result in almost all the deficiencies described above.